In a radiographic image processing apparatus, image processing is performed through intercepting a portion of direct radiation (direct rays) by a lead sheeting, etc., that intercepts radiation for suppressing decrease in contrast of a radiographic image due to influence of scattered radiation (scattered rays) or for avoiding unnecessary exposure to extract a radiation area. In this case, the radiographic image having undergone radiography needs to be optimized to have a suitable gradation. A radiation area needs to be extracted from the radiographic image for controlling a dose of radiation transmitting through a subject upon radiography.
Thus there exists a technique that a plurality of combinations of two candidate points is generated from an image acquired from a radiation detector, an evaluated value is obtained to each candidate, and the candidate with a higher value is extracted as a contour of a radiation area. See, for example, Patent Literature 1. There also exists a technique that characteristic amounts are extracted from a blurred image having a lower frequency than a radiographic image, and a contour of the radiation area by a collimator (also referred to as a “collimator edge” or “collimation edge”) is extracted. See, for example, Patent Literature 2. Here in the field of image processing, a process for extracting components for discrimination is called “characteristic extraction” in description of an image.
[Patent Literature 1] Japanese Patent No. 4280729
[Patent Literature 2] Japanese Patent Publication No. 2006-181362A
In Patent Literature 1 as a conventional technique, characteristic points are extracted from the entire image data by filtering, derivation, etc. Consequently, when noise is contained in the image, not only the necessary characteristic corresponding to an edge of the radiation area but also other many unnecessary characteristic points are to be detected. As a result, a problem of decreasing detection accuracy may arise.
The extracted edge points are classified into groups for selecting a characteristic. When many characteristic points are detected, combination of candidate points may increase in number, which leads to complicated calculation. Moreover, a collimator is mostly rectangular. As a result, the radiation area has a rectangular shape, and accordingly the edge of the radiation area is of straight line pattern. On the other hand, a problem also arises that even excessive patterns (e.g., straight line data on sides of a subject) may be detected.
This invention has been made having regard to the state of the art noted above, and its object is to provide a radiographic image processing apparatus and a radiographic image processing program that enables characteristic extraction and area extraction with high accuracy under reduced influences, such as noise, and decreased amounts of calculation.